1. Field of the Invention
The present invention is generally directed toward methods and devices for joining bodies and, more specifically, toward a method and device for integrally joining several bodies with joining elements.
2. Description of the Related Art
The economic joining of parts in industrial manufacturing is one of the main problems encountered. Especially porous material, such as wood, pressboard boards, porous stones or other materials are difficult to process. Apart from the conventional methods, which are based on mechanical fixing elements, other techniques are also known. Mentioned here in an exemplary manner for these shall be gluing together.
Thermal processes, which are based on the melting down of certain materials, are becoming more and more popular. In these processes, the surfaces to be joined are pressed together, e.g., under friction, so that either one of the base materials of the parts to be joined, or else a further material, is melted down due to the friction heat, as a result of which the parts are firmly joined together. The processes known today have various disadvantages. Mechanical joints, such as screws or nails, which primarily are based on frictional or positive locking engagement, in part call for very elaborate processing, they are subject to a high notch effect, they are easily torn out or else loosen over the course of time. Especially mentioned here shall be the problem of torn out or loosening fixing points in pieces of furniture made out of pressboard boards or similar materials. This has to be attributed to high stress concentrations and unfavorable load concentrations.
Nailing, which has been known from time immemorial, has a significant advantage: A nail can be processed easily and in a very short time and without requiring any additional preparation. The disadvantage, however, consists of the fact that also in the case of a nail the joint is one that is only based on frictional engagement and which, therefore, can only be subjected to a relatively small load. In addition, these joints, as the basis of the displacement of the base material, frequently lead to a cracking of the parts to be joined.
Integral joints with the materials in engagement, such as are produced, for example, by gluing, do not have the tendency, that the parts to be joined crack or split. They are characterized, however, by other disadvantages. These consist, for example in the long stoppage times, which are necessary when joining the parts, the low penetration depth of the adhesive into the parts to be joined or the difficult to control quantities of the adhesives (flowing away).
Thermal processes, which are based on the melting down of materials, with respect to an economical processing tangentially point in the right direction and they can roughly be divided into two groups. The first group consists in the method that, for example, the surfaces of two parts, one of which consists of a thermo-plastic plastic material, are pressed against one another and moved parallel (relative) to one another, so that friction heat is generated. The plastic material melted down by this frictional heat, after cooling down, forms an integral joint with material engagement between the two parts. This material engagement, however, is limited to the surface of the parts. The penetration depth of the material melted down and the application forces are always low, as a result of which these joints can only be subjected to very limited loads. A corresponding application is known, for example, from GB 2 061 183.
The second group of the thermal processes is based on the method that, for example, dowels or similar elements, typically made of meltable plastic material, are inserted into previously manufactured bores and are subsequently melted down by mechanical excitation and pressure. A corresponding method is known, for example, from PCT/EP95/02527. Significant in this is the fact that the parts to be joined for receiving the dowels imperatively have to be pre-drilled or pre-cleared before the dowels can be inserted into the bores and joined with the side walls at predefined points (certain zones at the end of the bore and along the dowel) by melting down. The thermal energy necessary for this is generated by radiation or by means of friction produced by ultrasonic excitation. Because of the necessary and precise pre-drilling, this is a process, which calls for several working steps.
PCT/CH98/00109 demonstrates a method for joining two bodies through a joining element. The joining element is inserted loosely into a bore. The joining element is subsequently partially made to melt down so that a superficial joint results.
From EP 0 268 957 a method for joining foils is known. In the case of this method, a sharpened joining element made of thermoplastic material is driven through a foil or plate after these have been brought to a partially plastic or ductile condition by heating. An embodiment illustrates a nail that is being driven into a non-thermoplastic substrate through a foil, which is in a partially plastic or ductile condition. The method is not suitable for joining larger parts.